Engines that burn ammonia gas have conventionally been proposed. For example, Japanese Patent Application Laid-Open No. 5-332152 discloses a technique for decomposing ammonia gas into hydrogen and nitrogen by the heat of exhaust gas from a combustion chamber and using the above hydrogen gas to efficiently burn ammonia gas that is separately supplied into the combustion chamber. Japanese Patent Application Laid-Open No. 2010-159705 discloses a technique used in an ammonia-burning internal combustion engine in which the latent heat of vaporization of liquid ammonia ejected into an intake port of a cylinder is used to reduce the temperature of intake air supplied into the combustion chamber.
Incidentally, there is a constant demand for improving energy efficiency in various types of engines. With engines including a cylinder and a piston, a reduction in the amount of work in a compression stroke contributes greatly to improving energy efficiency. However, with engines provided with a supercharger, high-pressure air compressed by the supercharger is supplied to the cylinder and causes an increase in the amount of compression work.